1) Field of the Invention
The present invention relates to an image forming apparatus and an optical scanner in the image forming apparatus to optically scan and write an image on an image carrier.
2) Description of the Related Art
In an optical scanner employed in an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multifunction product, optical parts such as a light source, a polygon mirror, various lenses, and a reflection mirror are provided to form an optical scanning path. A laser beam from the light source is reflected by the polygon mirror to perform deflection scanning. Deflected optical beam passes through a scanning lens, reflected by the reflection mirror, and emitted to outside of the optical housing to perform writing on an image carrier.
One example of such an optical scanner is explained, with reference to FIGS. 12 to 14. The optical scanner 21 includes a laser source 22 that emits a laser beam, a rotating mirror deflector 23 to deflect the laser beam emitted from the laser source 22, a scanning optical system 25 that forms a scanning path for scanning the peripheral surface of a photosensitive member 24, synchronous detecting units 26 and 27 to which a part of the laser beam deflected by the rotating mirror deflector 23 is irradiated as a synchronous detecting beam, and an optical housing 28 that houses and holds the laser source 22, the rotating mirror deflector 23, the scanning optical system 25, and the synchronous detecting units 26 and 27. The scanning optical system 25 includes lenses 25a and 25b, and a reflection mirror 25c. 
In this optical scanner 21, the laser beams emitted from the laser source 22 are deflected by the rotating mirror deflector 23 rotated at a high speed, and the scanning path is formed from the deflected laser beams by the scanning optical system 25. This scanning path scans the peripheral surface of the photosensitive member 24, to thereby form an electrostatic latent image on the peripheral surface of the photosensitive member 24. This electrostatic latent image is developed by a toner to become a toner image, and the toner image is transferred onto a recording medium to perform image forming onto the recording medium. A part of the laser beams deflected by the rotating mirror deflector 23 is detected as the synchronous detecting beam by the synchronous detecting units 26 and 27. Based on a result of the detection, a scanning start position and a scanning end position of the scanning path are restricted.
In such an optical scanner, pixel density in the horizontal scanning direction is determined by a write clock frequency and the horizontal scanning speed of the lens optical system. It is important to accurately maintain the write density for obtaining excellent image quality. In the optical scanner having a plurality of optical scanning paths like the color image forming apparatus, if the pixel density is different for each optical scanning path, when images written by using the respective optical scanning paths are superposed, out-of-color registration occurs. Therefore, it is especially important to accurately maintain the write density.
Accordingly, in the conventional optical scanners, there is one in which a pair of optical detecting units is arranged on the write-start position side and the write-end position side outside the image area of the optical scanning path, to measure the horizontal scanning time between the two points by the detection timings, a variation quantity from the aimed horizontal scanning magnification is calculated from the measurement result, and the write clock frequency is changed according to the variation quantity, to thereby correct an error in the horizontal scanning magnification.
In this type of optical scanner, however, since a pair of optical detecting units is directly fixed on the optical housing, the distance between the pair of optical detecting unit changes due to a thermal deforming of the optical housing by a change in the ambient temperature, and hence accurate scanning time cannot be obtained.
The cause of thermal deforming occurring in the optical housing is not only the heat from the rotating mirror deflector, but also the heat from a heat source provided in the image forming apparatus, for example, a fixing unit and a power source unit. In other words, when the optical scanner is fixed in the image forming apparatus, a difference in thermal expansion occurs in the optical housing between the side close to the fixing unit, being the heat source, and the side away from the fixing unit, thereby causing a misalignment of the synchronous detecting unit close to the heat source, and the synchronous detecting unit away from the heat source.
Further, the temperature rise in the optical housing affects not only the misalignment of the scanning path, but also the beam spot diameter of the scanning path exposed on the surface of the photosensitive member. That is, the refractive index of the lenses constituting the scanning optical system changes due to the temperature change in the optical housing, and a predetermined beam spot diameter cannot be obtained, thereby causing image deterioration due to thickening of the beam spot diameter.
On the other hand, the mounting location of the synchronous detecting unit should be fixed at all times, but the position thereof slightly changes periodically, due to vibrations of the optical housing by the rotation of the rotating mirror deflector, or due to propagation of vibration from the driving unit of the image forming apparatus. As a result, the scanning start position and the scanning end position of the respective scanning paths change periodically, thereby causing image deterioration referred to as a so-called image fluctuation of longitudinal line.
Therefore, an invention in which variations in the distance between two optical detecting units due to a temperature change are eliminated as much as possible, to perform correction to a precise write clock frequency, so as to obtain a precise write density in the horizontal scanning direction has been disclosed (see, for example, Japanese Patent Application Laid-open No. H8-76038).
Such an optical scanner will be explained based on FIG. 15. Like reference signs designate like parts as in FIGS. 12 to 14, and explanation thereof is omitted. This optical scanner 21a has the same components as those of the optical scanner 21 explained with reference to FIGS. 12 to 14, and the different point is the arrangement position of the synchronous detecting units 26a and 27a. In the optical scanner 21 explained with reference to FIGS. 12 to 14, the synchronous detecting units 26 and 27 are arranged on the opposite side of the rotating mirror deflector 23, with the scanning optical system 25 put therebetween, and are away from the rotating mirror deflector 23 and the laser source 22. On the other hand, in the optical scanner 21a, the synchronous detecting units 26a and 27a are arranged at a position close to the rotating mirror deflector 23 and the laser source 22.
As described above, the optical scanner in the Patent Literature 1 is an optical scanner in which a pair of optical detecting units is arranged on an optical scanning path, to measure the scanning time from the write-start position to the write-end position, wherein at least one of the optical detecting units is fixed to the optical housing, via an intermediate member having a thermal expansion coefficient larger than that of the optical housing. The thermal deforming quantity of the optical housing and the thermal deforming quantity of the intermediate member are counterbalanced, so that a variation in the distance between the optical detecting units due to the influence of heat is reduced.
Further, an invention in which even when the heat generation state is different between the housing in which a scanning optical system is housed and a drum support in which a photosensitive drum is housed, the scanning start position and the scanning end position of the scanning path with respect to the respective photosensitive drums can be made uniform has been disclosed (see, for example, Japanese Patent Application Laid-open No. 2000-258715).
However, as an actual problem, counterbalancing of the thermal deforming quantity of the optical housing and the thermal deforming quantity of the intermediate member is not so easy. Further, in the optical scanner having a plurality of optical scanning paths, the optical housing and the intermediate member in the optical scanning paths do not always generate the same thermal deforming, and a misalignment occurs between the optical scanning paths due to a difference in the thermal deforming. Particularly, in the case of a color image forming apparatus, there is a problem of causing out-of-color registration.
This problem will be specifically explained, with reference to FIGS. 12 to 15. The rotating mirror deflector 23 and the laser source 22 are members generating heat when being driven, and due to heat generation of the rotating mirror deflector 23 and the laser source 22, the optical housing 28 thermally expands and deforms.
However, the deforming due to the thermal expansion of the optical housing 28, and the arrangement of the synchronous detecting units 26 and 27 (26a and 27a) are not taken into consideration.
Therefore, as shown in FIG. 13, the optical housing 28 deforms from the position indicated by the actual line to the position indicated by the broken line due to thermal expansion, and the synchronous detecting units 26 and 27 misalign with the deforming, and the synchronous detecting beams detected by the synchronous detecting units 26 and 27 change from “A” to “A′”, due to the misalignment. In other words, the detection timing of the synchronous detecting beam by the synchronous detecting units 26 and 27 changes, and the scanning time of the scanning path for scanning the photosensitive member 24 extends from “t” to “t+Δt”. In the image formed on a recording medium, the length in the horizontal scanning direction changes before and after the thermal expansion of the optical housing 28, thereby causing a magnification error. The same thing applies in the optical scanner 21a shown in FIG. 15.
As shown in FIG. 14, since the synchronous detecting beam “A” is made to enter obliquely with respect to the synchronous detecting unit 27, the light amount per unit area of the synchronous detecting beam “A” on the synchronous detecting unit 27 decreases. As a result, the incidence detection timing is likely to be unstable, thereby deteriorating the detection accuracy of synchronous detection, and causing a variation in the scanning time of the scanning path.
As shown in FIG. 15, when the synchronous detecting units 26a and 27a are located close to the rotating mirror deflector 23 and the laser source 22, the positions where the synchronous detecting units 26a and 27a are arranged in the optical housing 28 are likely to deform, due to the influence of heat from the rotating mirror deflector 23 and the laser source 22. Therefore, a misalignment of the synchronous detecting units 26a and 27a due to the deforming of the optical housing 28 due to thermal expansion, and a change in timing of detecting the synchronous detecting beam by the synchronous detecting units 26a and 27a due to the misalignment becomes conspicuous.